With conventional fuel injectors for injection systems of internal combustion engines the fuel injection nozzle is typically controlled by means of a valve needle, which is arranged to form a moving seal within a guide in a valve body of an injection valve. The valve needle features a valve needle seat at its tip, which, together with a valve body seat of the valve body, opens or closes at least one spray orifice to the combustion chamber of the internal combustion engine. The at least one spray orifice is typically located in the area of the valve body seat.
With the pressures which arise, the sealing edges and passages in the fuel injector are subject to significant mechanical stress, which also varies greatly during operation. The sealing of the injection valve is therefore especially important, even with common-rail systems, since by contrast with conventional systems for periodic injection, these are permanently under system pressure in each case so that bad seals can lead to continuous injection.
A seal between the pressure chamber and the leakage chamber can only be made between spray valve body and axially moving spray valve needle by means of a sealing gap, since any type of conventional elastic seal would not be able to withstand the high pressure loads over the long term. The sealing gap must be manufactured very precisely in order to be able, even with variations in temperature and the resulting material expansions, to simultaneously guarantee smooth guidance of the valve needle and thereby the best possible sealing effect.
An additional problem is caused by the fact that, with a common-rail injector there is a valve control piston to control the valve needle present in addition to the valve needle of the injection valve, which is also sealed by a gap seal against the exhaust chamber. The result of constantly high pressure on a number of gap seals is a relative large volume of leakage which adversely affects the overall efficiency of the system. One way of avoiding this leakage is to reduce the guide play to a low figure of 1.5 to 2.5 μm between valve needle seat and valve body. A further measure consists of coating the needle in the area of its guide, in a form of plastic coatings for example. As large a guide length as possible (>10 mm) of the valve needle can also reduce leakage. The play in the guide is however limited by manufacturing tolerances and by the requirements of lubrication and the guide length by the maximum dimensions of the injector.